Suppress RuboCop's offense

[koic]

Follow rubocop/rubocop#8707

This PR suppresses the following RuboCop's offense.

% bundle exec rake
(snip)

Offenses:

spec/rubocop/cop/active_record_helper_spec.rb:6:3: W:
Lint/ConstantDefinitionInBlock: Do not define constants within a block.
  module RuboCop ...
    ^^^^^^^^^^^^^^

185 files inspected, 1 offense detected, 1 offense auto-correctable
RuboCop failed!

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.